sensor ii.ppt

EN 2850 APLLIED ELECTRONICS

LECTURE 03 – Sensors(2)

Level 2/Semester 129/09/2009Kosala Jayasundara

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Light (photo) detectors

Detectors used to sense the electromagnetic radiation in the spectral range from UV to far infrared belongs to this category.

This can be used either to sense the presence/absence of light or to sense the intensity of light.

Quantum type of light detectors are operating up to mid infrared region of the spectrum.

Presence of electromagnetic energy (light) can cause some electrical properties of materials to be changed.

This phenomenon of photo effect was discovered by A.

Einstein.

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Light (photo) detectors (Contd…)

Three classes of light detectors can be identified. Photoconductive – The electrical conductivity of the

material changes as a function of the incident light intensity. When light falls on to the material it generates free electrons which leads to this conductivity change.

Photovoltaic – These type of devices contain a semiconductor junction (p-n) and a voltage is generated upon incident of light.

Photoelectric – These devices are based on photo electric effect where incident photons release electrons from the surface of the detector and then collected through an external circuit.

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Light Dependent Resistor – LDRThis is a type of a resistor whose

resistance decreases with the increase of incident light. This is a photo conductive type device usually made out from high resistance semiconductor material. (CdS or CdSe)

When the light falls on to the material, photons are absorbed by the semiconductor causing the electrons to jump in to the conduction band from the valance band. The resulting free electrons and holes contribute to the conduction, causing the resistance to be reduced.

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It can be shown that for a lower cell resistance and better sensitivity the length between the electrodes (leads) should be minimized while the breadth of the sensor is maximized. To achieve this, the LDR is fabricated in the above shown shape – serpentine shape

Sensitivity = No of electrons generated per unit photon incident

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LDR is widely used to detect the presence or absence of light.

One disadvantage of LDR is slow response time.

Dark activated Light activated

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Photo diode – This is a type of semiconductor diode used to detect light. When a p-n junction is forward biased the conduction is dominated by the majority carriers and the majority carrier concentration depends on the doping. But when the junction is reversed biased the minority carriers in both sides crosses the junction causing a reverse current to flow. Since the minority carrier concentration depends on the electron hole pair generation it can be controlled by heat or light. This is the principle behind the photo diode.

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Two modes of operation Photoconductive region – The junction is reversed

biased and with the increase of the incident light intensity the minority carrier concentration increases causing the reverse current to be increased.

Photovoltaic region – The junction is zero or forward biased. Used as photocell.

Photodiode operates in photoconductive region when used as a light detector. The generated reverse current (photo current) is proportional to the incident electromagnetic power and the proportionality constant is called the responsivity of the photo diode.

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Temperature detectors

The effect of thermal energy over the electrical properties of materials, is utilized to sense the temperature.

When there is a temperature difference between two points thermal energy flows either by,

Conduction Convection Radiation. or combination of these.

Thus two main types of temperature detectors are available. Contact temperature sensors and Non -contact temperature sensors.

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Temperature detectors (Contd…)

Contact type Non -contact type

Conduction, Convection

Radiation

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Themistors – This is a temperature sensitive resistor usually made out from copper or nickel like metals or metal oxides. This is essentially an active sensor where the change in resistance has to be converted to a voltage variation.

When the material is subjected to increase in temperature (incident thermal energy) two things can happen.

When the lattice structure absorbs energy the lattice vibration increases causing the opposition to the electron flow (resistance) to be increased.

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When the valance electrons absorbs energy they can be driven in to conduction band causing the carrier density to be increased, hence decreasing resistance.

Depending on the actual material used, one of these two phenomenon dominates.

Hence two types of themistors are available NTC – Negative Temperature Coefficient

Resistance decreases as temperature increases.

PTC – Positive Temperature Coefficient

Resistance increases as the temperature increases.

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NTC

PTC

Made from thin coil of semiconducting materials such as sintered metal oxideCan be either bead type or chip type

Self heating issue

Metallic materials like copper and nickel are used.

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Thermocouple - Based on the principle that when a conductor is subjected to a thermal gradient a voltage will be garneted. In order to measure this voltage an additional conductor at the two ends is needed. This additional conductor will be then experience the thermal gradient and develop a voltage of its own, that will oppose the first one. Thus by completing the circuit by different materials a differential voltage can be made available for measurement. Usually this differential voltage is in the order of several micro-volts per degree Celsius.

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Thermocouple is essentially a passive sensor. Cheap and can be used to measure a wide

range of temperatures. Accuracy limitation – errors of less than 10C is

difficult to achieve. Thermocouple is a relative sensor.

Insulated thermocouple

Butt-Welded thermocouple

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Force, Strain detectors

Force sensors can either be quantitative or qualitative

Quantitative force sensors measure the force and represents its value as an electrical signal. (E.g. Strain gauges and load cells)

Qualitative force sensors only detects whether the force exceeds a given threshold. These are commonly used as position detectors.

There are various methods that can be used to measure/sense the force.

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Force, Strain detectors (Contd…)

Force

Balance the force to be measured with an electromagnetically

developed force

Convert the force in to a fluid pressure and

measure the pressure.

Measure the strain produced by the forced

under measurement, in an elastic member.

Balance the force under measurement

with a known gravitational force.

Measure the acceleration of a known mass caused by the force under measurement.

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Force, Strain detectors (Contd…)

Strain gauge – Make use of the property that the resistance varies upon an applied stress.

It can be shown that the change of the resistance per unit change in the length is proportional to the initial length of the conductor.

Thus the sensitivity can be increased by having a large initial length. Therefore practically they are arranged to facilitate this.

Un-axial strain gauge.

Direction of sensing

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Chemical sensors

These are sensors that generate electrical output upon a stimuli produced by chemicals or chemical reactions.

Chemical sensors are used in wide range of applications such as air pollutant measurement, exhaust gas controlling in modern vehicles, bio medical applications, chemical based process control application etc.

Chemical sensors can either be direct type sensors or complex sensors.

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Chemical sensors (Contd…)

Sensor Transducer Sensor

Direct type Complex type

Chemical phenomena

Electrical signal

Chemical phenomena

Electrical signal

E.g. Metal oxide type sensors. When hydrocarbons are present measurable drop in resistance is observed

Elastomer chemiresistors

E.g. Pellister – Gases act on the device and reaction takes place and heat is produced. Thermal sensor is then used to obtain an electrical output.

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Next lecture …

Position, displacement and level detectors

Motion detectors Example – how to select the sensors for

a given application

sensor ii.ppt

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